A fuel cell provides electricity and heat at the same time by electrochemically reacting a fuel gas such as hydrogen with an oxidant gas such as air in catalytic layers of electrodes. Fuel cells offer the possibility of converting chemically bonded energy directly into electrical energy which can be subsequently converted into mechanical drive energy with the aid of an electric motor. Present day preferred fuel cells consume hydrogen and oxygen and convert these elements into the environmentally friendly end product, namely, water.
Hydrogen is the most direct fuel of fuel cell, that is, it can be used without the conversion of reformer. Hydrogen is a substance with high energy content compared to its weight. On the other hand, the energy content compared to volume is rather low. This poses greater challenges with respect to storage compared to storage of gasoline which is a liquid. There are basically three options:                hydrogen may be compressed and stored in a pressure tank, such as high-pressure hydrogen storage steel bottle.        hydrogen may be cooled to a liquid state and kept cold in a properly insulated tank, such as liquid hydrogen storage tank.        hydrogen may be stored in a solid compound, such as metal hydride canister and carbon nanofibers.With respect to storing density, convenience, safety, and cost, the metal hydride canister will be the preferred option.        
FIG. 1 is a cross-sectional view illustrating a metal hydride tank apparatus according to ROC Pat. No. 491336. Please refer to FIG. 1, the metal hydride tank apparatus 100 mainly comprises: a metal hydride 102, a shell 104 containing the metal hydride 102, a gas-valve controlling unit 106, and a heat exchanger 108 arranged inside the shell 104, wherein, the top of the shell 104 has an opening 110 having two holes (not shown) arranged at the circumferential surface thereof, and the gas-valve controlling unit 106 is arranged within the opening 110.
As seen in FIG. 1, the heat exchanger 108 further comprises: a first end 112 passing through one of the hole arranged at the circumferential surface of the opening 110; a second end 116 passing through another hole arranged at the circumferential surface of the opening 110; and a single spiral shaped middle section 114 located inside the shell 104. Therefore, when it is desired to promote an exothermic reaction in the metal hydride 102 for storing hydrogen or to promote an endothermic reaction in the metal hydride 102 for discharging hydrogen, the crooked spiral-shaped middle section of the heat exchanger 108 having a large surface contacting with the metal hydride 102 is capable of speeding up the reaction. The gas-valve controlling unit 106 of the prior metal hydride tank apparatus 100 further has a filter 118, and the filter 118 is made of palladium using powder sinter molding method, for allowing the flowing of hydrogen and filtering out other gases and impurities.
However, during the hydrogen absorbing and discharging procedures, the conventional metal hydride tank apparatus is not ideal for the following reasons:
1. When hydrogen quickly enters into and exhausts from the conventional metal hydride tank apparatus, the flow rate of the hydrogen will be quickly slowed down, because of the heat absorption and the heat release of the metal hydride.
2. If the thickness of the filter is not thick enough, then the ability for filtering out the impurities and other gases will be insufficient.
3. If the thickness of the filter is too thick, then the resistance for discharging the hydrogen will be increased.
4. Because the contact area between the single spiral pipe of the heat exchanger and the metal hydride is not large enough, it is impossible to heat the metal hydride uniformly or absorb the heat released by the metal hydride uniformly, such that the hydrogen absorbing and discharging capability of the metal hydride is affected.